DESCRIPTION: (adapted verbatim from the investigator's abstract) The development of mouse models containing a p53 null allele has been invaluable in deciphering the importance of the p53 tumor suppressor in tumorigenesis. However, greater than 80% of p53 alterations are misense mutations in the DNA binding domain and as such, the p53 null mouse does no represent the majority of mutations that occur. The hypothesis to be tested in this proposal is that missense mutations represent a dominant-negative or gain-of-function phenotype in vivo. We have generated a mouse containing an arg-to-his substitution in the endogenous p53 gene corresponding to one of six hotspot mutations (amino acid 175) in human tumors. This mouse also contains the deletion of a G nucleotide at the splice acceptor site of exon 3 and expresses the mutant protein at levels similar to wild type p53. Another mouse containing the arg-to-his mutation expressed at higher levels will also be made. In order to test the dominant-negative and gain-of-function nature of this mutation, we will compare the p53R172H mutations in the presence or absence of a p53 null allele for timing and spectrum of tumor development. Another mutation found in human tumors at this amino acid alters arginine 175 to proline. This mutant retains the ability to arrest growth in G1, but not to induce apoptosis when assayed in tissue culture. ES cells containing this point mutation have been identified and a mouse model will be developed. This model will be crucial in understanding the relative contribution of p53 growth arrest and apoptotic functions to tumorigenesis in vivo. The p53R172H and p53R172P mice will also be mated with mdm2 heterozygous mice to monitor effects on embryo lethality due to loss of mdm2. The possibility of delaying lethality of mdm2 null mice will afford the opportunity to examine the importance of the p53/MDM2 interaction in later development. These mouse models will yield insight into differences between p53 missense mutations in the timing and spectrum of tumor development.